MOLKCULAk ROTATION 119 illumination in the two halves of the field, the nieol N must be turned through an angle equal to the angle of rotation, which is then measured on the divided circle. When the angle « is small, i.e. when the plane of vibration of the polarised light is almost parallel to the optic axis of the quartz, the greatest degree of sensitiveness is attained, for then a very small change in the position of N causes a great difference in the respective illuminations in the two halves of the field. As « increases, the sensitiveness diminishes, but a greater total intensity of illumination is ob- tained. By moving j (Fig. 71) the position of the nicol p may be altered. For clear colourless liquids the angle a may be made comparatively small ; but in the case of coloured liquids it is necessary to have a larger, and so obtain a greater intensity of light at the cost of sensitiveness. Calculation of Results ; Homogeneous Liquids.- The angle of rotation, represented by «i> (for sodium light), varies with the length of the column of substance through which the light passes. One decimetre has been chosen as unit of length. The angle also varies with the temperature, which must conse- quently be determined for each observation. For the comparison of the rotary power of different substances, use is made of the constant specific rottition, which may be defined as the angle of rotation, produced by I gram of active substance in I c.c. by a layer I dm. in length. This is obtained by dividing the observed angle of rotation by the product of the length in decimetres, and the density of the substance at the temperature at which the observation was made. Molecular Rotation is the above quantity multiplied by the molecular weight M of the compound, and divided by 100 to avoid unwieldy numbers, and is represented thus ......... . 100 It expresses the angle of rotation of i mm. of active substance containing i gram-molecule in i c.c.